1. Field of the Invention
The present invention relates to a process for the preparation of electrochemical catalysts of fuel cells based on polymer electrolytes with excellent electrochemical catalysts activity by uniformly supporting a large amount of nano-sized metal particles on the carbon support surface without agglomeration.
2. Description of Related Art
A fuel cell is an apparatus that converts chemical energy of fuel into electric energy by the electrochemical reaction of fuel, such as hydrogen or methanol, and oxygen. The fuel cell has been spotlighted as a next generation clean energy source because of the high generation efficiency without suffering from a carnot cycle and the lower emission of pollutants such as NOX, SOX than the existing thermal power generations. It also does not generate noise during operation. The fuel cell is sorted into a polymer electrolyte fuel cell (PEMFC), a phosphoric acid fuel cell (PAFC), a molten carbonate fuel cell (MCFC), a solid oxide fuel cell (SOFC), etc., according to an electrolyte used. Among others, the polymer electrolyte fuel cell has features of lower operating temperature, better generation efficiency, and more compact than the other fuel cells, therefore, it is widely used as a power supply for an electric vehicle, a small power plant for a house, a mobile emergency power supply, and a power supply for a military, etc.
The polymer electrolyte fuel cell is typically formed in a five-layer structure of collector/fuel electrode/polymer electrolyte membrane/air electrode/collector. The fuel cell is supplied with fuel such as hydrogen or methanol and the air electrode is supplied with air or oxygen. The fuel is oxidized in the fuel electrode to generate protons and electrons. At this time, the protons move to the air electrode through the electrolyte membrane and the electrons move to the air electrode through wires and loads configuring an external circuit. The reduction reaction of protons and electrons with oxygen is performed in the air electrode generating water, wherein water is then discharged from the fuel cell.
Both electrodes of the polymer electrolyte fuel cell are manufactured by forming a catalyst layer on the carbon species or carbon clothes through an application of an ink made of a catalyst for the activation of the oxidation-reduction reaction, a polymer electrolyte, and a solvent. For a catalyst, a platinum-based catalyst such as platinum or an alloy of platinum/ruthenium with very excellent catalyst activity in carbon particulate support has been in practical use.
The catalyst for the polymer electrolyte fuel cell should have characteristics, such as a large specific surface area of metal particle, strong adhesion between the metal particle and the catalyst support, improved CO tolerance, chemical uniformity between metal atoms upon alloying, etc. In particular, a large amount of metal particles should be uniformly supported on a surface of the carbon particulate without agglomeration so that the electrochemical catalyst activity becomes high. Also, for the early commercialization of the fuel cell, a manufacturing process of the catalyst should be inexpensive and environment-friendly.
As a conventional process of manufacturing the catalyst, an absorbent reduction process, which mixes an aqueous solution of platinum compound or an aqueous solution of platinum compound and ruthenium compound with a carbon powder as a catalyst support and disperses the mixture, precipitates particles by adding a reducing agent, such as sodium borohydride (NaBH4), alcohol, aldehyde, etc., thereto, or reduces it by performing an annealing under a hydrogen atmosphere, has been generally used. This process is ease and convenient, but when a supported amount of metal particles is increased, it is difficult to support them uniformly without agglomeration and also hard to control the particle size.
As an alternative process, the colloid method was proposed by Watanabe (J. Electroanal. Chem., 229, 1987, 395), etc. This process can uniformly precipitate particles on a colloid using NaHSO3, H2O2 as a reducing agent. However, this process has a difficulty in controlling a reaction condition such as pH and since a particle phase is precipitated in an oxide form, it requires a high-temperature hydrogen annealing, thereby increasing its manufacturing cost. As a result, this process is not proper for the mass production.
Another process, Bonnemann process (Angew. Chem., 103, 1991, 1344)—prepares the catalysts by synthesizing metal particles stabilized through surfactant followed by the attachment of them to carbon support. This process is not suitable for the mass production due to a use of toxic solvent, tetrahydrofuran (THF), a need of the high-temperature hydrogen annealing treatment like the Watanabe process, and a relatively weak adhesion between the metal particle and the carbon support.
In addition to the aforementioned processes, a pyrolysis process (Xing et al., Chem. Comm., 12, 2005, 1601.)—supports metal particles by pyrolyzing an aerosol made of support and metal compound by allowing it to pass through a reactor in a high-temperature reduction atmosphere state using a carrier gas, or a vapor phase synthesis process such as a combustion chemical vapor deposition process (Yu Ji Bum, Korea patent application No, 10-2004-0025987) prepares a catalyst by using support as a substrate and performing a chemical vapor reaction of metal compound on the support. Since these processes have problems in that the yield of the catalyst is low and the supported amount of metal particles is restricted, it is known that they are not proper for mass production.
Meanwhile, many studies on the improvement of the support, which is another component configuring the catalyst, have been progressed. Since the support firmly supports the metal particles and also severs as a path for rapidly moving electrons generated upon performing the oxidation-reduction reaction to the collector, it should be strongly adhered to the metal particles and have excellent electrical conductivity. As usual support, carbon black with a large specific surface area such as Vulcan-XC or Ketjen Black has been used. Recently, for the purpose of increasing catalyst availability, carbon fiber or carbon nanotube having excellent electrical conductivity due to a large aspect ratio and a crystal structure of a graphite phase has been actively studied. However, when using the carbon fiber or the carbon nanotube as the support, the most serious problems are that it is difficult to disperse the support and to support a large amount of metal particles due to a relatively small specific surface area and also it is easy to generate an agglomeration between the metal particles